[FEATURE] In tree mode, add a button to hide empty folders

[chtixof]

I like the tree representation mode. I understood that it is "work in progress". However, I think it can help to have this feedback in the form of a feature request.

In the tree mode, when a tag is selected, it is not convenient to find the tagged notes, because of the folders not containing tagged notes (= those folders are empty for the tag, and not relevant).

So, there should be a button or check box telling to hide the empty folders, so we can focus on the folders actually containing notes for the selected tag.

There could be also another button or check box telling to display all the notes in a flat list, without the folder organisation.

[pbek]

We can link that to #790.